time: record monotonic clock reading in time.Now, for more accurate comparisons

See https://golang.org/design/12914-monotonic for details.

Fixes #12914.

Change-Id: I80edc2e6c012b4ace7161c84cf067d444381a009
Reviewed-on: https://go-review.googlesource.com/36255
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Caleb Spare <cespare@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/archive/tar/tar_test.go

@@ -97,10 +97,13 @@ func TestRoundTrip(t *testing.T) {
 		Name: "file.txt",
 		Uid:  1 << 21, // too big for 8 octal digits
 		Size: int64(len(data)),
-		ModTime: time.Now(),
+		// AddDate to strip monotonic clock reading,
+		// and Round to discard sub-second precision,
+		// both of which are not included in the tar header
+		// and would otherwise break the round-trip check
+		// below.
+		ModTime: time.Now().AddDate(0, 0, 0).Round(1 * time.Second),
 	}
-	// tar only supports second precision.
-	hdr.ModTime = hdr.ModTime.Add(-time.Duration(hdr.ModTime.Nanosecond()) * time.Nanosecond)
 	if err := tw.WriteHeader(hdr); err != nil {
 		t.Fatalf("tw.WriteHeader: %v", err)
 	}

src/encoding/gob/gobencdec_test.go

@@ -746,7 +746,7 @@ func (i *isZeroBugInterface) GobDecode(data []byte) error {
 }
 
 func TestGobEncodeIsZero(t *testing.T) {
-	x := isZeroBug{time.Now(), "hello", -55, isZeroBugArray{1, 2}, isZeroBugInterface{}}
+	x := isZeroBug{time.Unix(1e9, 0), "hello", -55, isZeroBugArray{1, 2}, isZeroBugInterface{}}
 	b := new(bytes.Buffer)
 	enc := NewEncoder(b)
 	err := enc.Encode(x)

src/go/types/testdata/expr3.src

@@ -207,7 +207,7 @@ func struct_literals() {
 	_ = time.Time{}
 	_ = time.Time{sec /* ERROR "unknown field" */ : 0}
 	_ = time.Time{
-		0 /* ERROR implicit assignment to unexported field sec in time.Time literal */,
+		0 /* ERROR implicit assignment to unexported field wall in time.Time literal */,
 		0 /* ERROR implicit assignment */ ,
 		nil /* ERROR implicit assignment */ ,
 	}

src/runtime/stubs.go

@@ -241,8 +241,7 @@ func stackBarrier()
 // in asm_*.s
 func return0()
 
-//go:linkname time_now time.now
-func time_now() (sec int64, nsec int32)
+func walltime() (sec int64, nsec int32)
 
 // in asm_*.s
 // not called directly; definitions here supply type information for traceback.
@@ -281,7 +280,7 @@ func prefetcht2(addr uintptr)
 func prefetchnta(addr uintptr)
 
 func unixnanotime() int64 {
-	sec, nsec := time_now()
+	sec, nsec := walltime()
 	return sec*1e9 + int64(nsec)
 }
 

src/runtime/sys_darwin_386.s

@@ -217,8 +217,8 @@ inreg:
 	ADCL	$0, DX
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$0
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$0
 	CALL	runtime·now(SB)
 	MOVL	$1000000000, CX
 	DIVL	CX

src/runtime/sys_darwin_amd64.s

@@ -198,8 +198,8 @@ TEXT runtime·nanotime(SB),NOSPLIT,$0-8
 	MOVQ	AX, ret+0(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$0-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$0-12
 	CALL	nanotime<>(SB)
 
 	// generated code for

src/runtime/sys_darwin_arm.s

@@ -159,7 +159,7 @@ TEXT runtime·mincore(SB),NOSPLIT,$0
 	MOVW	R0, ret+12(FP)
 	RET
 
-TEXT time·now(SB), 7, $32
+TEXT runtime·walltime(SB), 7, $32
 	MOVW	$8(R13), R0  // timeval
 	MOVW	$0, R1  // zone
 	MOVW	$0, R2	// see issue 16570

src/runtime/sys_darwin_arm64.s

@@ -151,7 +151,7 @@ TEXT runtime·setitimer(SB),NOSPLIT,$0
 	SVC	$0x80
 	RET
 
-TEXT time·now(SB),NOSPLIT,$40-12
+TEXT runtime·walltime(SB),NOSPLIT,$40-12
 	MOVD	RSP, R0	// timeval
 	MOVD	R0, R9	// this is how dyld calls gettimeofday
 	MOVW	$0, R1	// zone

src/runtime/sys_dragonfly_amd64.s

@@ -148,8 +148,8 @@ TEXT runtime·setitimer(SB), NOSPLIT, $-8
 	SYSCALL
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVL	$232, AX // clock_gettime
 	MOVQ	$0, DI  	// CLOCK_REALTIME
 	LEAQ	8(SP), SI

src/runtime/sys_freebsd_386.s

@@ -159,8 +159,8 @@ TEXT runtime·setitimer(SB), NOSPLIT, $-4
 	INT	$0x80
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVL	$232, AX // clock_gettime
 	LEAL	12(SP), BX
 	MOVL	$0, 4(SP)	// CLOCK_REALTIME

src/runtime/sys_freebsd_amd64.s

@@ -142,8 +142,8 @@ TEXT runtime·setitimer(SB), NOSPLIT, $-8
 	SYSCALL
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVL	$232, AX // clock_gettime
 	MOVQ	$0, DI		// CLOCK_REALTIME
 	LEAQ	8(SP), SI

src/runtime/sys_freebsd_arm.s

@@ -166,8 +166,8 @@ TEXT runtime·setitimer(SB), NOSPLIT, $-8
 	SWI $0
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVW $0, R0 // CLOCK_REALTIME
 	MOVW $8(R13), R1
 	MOVW $SYS_clock_gettime, R7

src/runtime/sys_linux_386.s

@@ -151,8 +151,8 @@ TEXT runtime·mincore(SB),NOSPLIT,$0-16
 	MOVL	AX, ret+12(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVL	$265, AX			// syscall - clock_gettime
 	MOVL	$0, BX		// CLOCK_REALTIME
 	LEAL	8(SP), CX

src/runtime/sys_linux_amd64.s

@@ -135,8 +135,8 @@ TEXT runtime·mincore(SB),NOSPLIT,$0-28
 	MOVL	AX, ret+24(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$16
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	// Be careful. We're calling a function with gcc calling convention here.
 	// We're guaranteed 128 bytes on entry, and we've taken 16, and the
 	// call uses another 8.

src/runtime/sys_linux_arm.s

@@ -197,7 +197,7 @@ TEXT runtime·mincore(SB),NOSPLIT,$0
 	MOVW	R0, ret+12(FP)
 	RET
 
-TEXT time·now(SB), NOSPLIT, $32
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVW	$0, R0  // CLOCK_REALTIME
 	MOVW	$8(R13), R1  // timespec
 	MOVW	$SYS_clock_gettime, R7

src/runtime/sys_linux_arm64.s

@@ -182,8 +182,8 @@ TEXT runtime·mincore(SB),NOSPLIT,$-8-28
 	MOVW	R0, ret+24(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$24-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$24-12
 	MOVW	$0, R0 // CLOCK_REALTIME
 	MOVD	RSP, R1
 	MOVD	$SYS_clock_gettime, R8

src/runtime/sys_linux_mips64x.s

@@ -172,8 +172,8 @@ TEXT runtime·mincore(SB),NOSPLIT,$-8-28
 	MOVW	R2, ret+24(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$16
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	MOVW	$0, R4 // CLOCK_REALTIME
 	MOVV	$0(R29), R5
 	MOVV	$SYS_clock_gettime, R2

src/runtime/sys_linux_mipsx.s

@@ -175,8 +175,8 @@ TEXT runtime·mincore(SB),NOSPLIT,$0-16
 	MOVW	R2, ret+12(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	MOVW	$0, R4	// CLOCK_REALTIME
 	MOVW	$4(R29), R5
 	MOVW	$SYS_clock_gettime, R2

src/runtime/sys_linux_ppc64x.s

@@ -157,8 +157,8 @@ TEXT runtime·mincore(SB),NOSPLIT|NOFRAME,$0-28
 	MOVW	R3, ret+24(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$16
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	MOVD	$0, R3 // CLOCK_REALTIME
 	MOVD	$0(R1), R4
 	SYSCALL	$SYS_clock_gettime

src/runtime/sys_linux_s390x.s

@@ -169,8 +169,8 @@ TEXT runtime·mincore(SB),NOSPLIT|NOFRAME,$0-28
 	MOVW	R2, ret+24(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$16
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	MOVW	$0, R2 // CLOCK_REALTIME
 	MOVD	$tp-16(SP), R3
 	MOVW	$SYS_clock_gettime, R1

src/runtime/sys_nacl_386.s

@@ -233,7 +233,7 @@ TEXT runtime·mmap(SB),NOSPLIT,$32
 	MOVL	AX, ret+24(FP)
 	RET
 
-TEXT time·now(SB),NOSPLIT,$20
+TEXT runtime·walltime(SB),NOSPLIT,$20
 	MOVL $0, 0(SP) // real time clock
 	LEAL 8(SP), AX
 	MOVL AX, 4(SP) // timespec
@@ -249,7 +249,7 @@ TEXT time·now(SB),NOSPLIT,$20
 	RET
 
 TEXT syscall·now(SB),NOSPLIT,$0
-	JMP time·now(SB)
+	JMP runtime·walltime(SB)
 
 TEXT runtime·nacl_clock_gettime(SB),NOSPLIT,$8
 	MOVL arg1+0(FP), AX

src/runtime/sys_nacl_amd64p32.s

@@ -242,7 +242,7 @@ TEXT runtime·mmap(SB),NOSPLIT,$8
 	MOVL	AX, ret+24(FP)
 	RET
 
-TEXT time·now(SB),NOSPLIT,$16
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	MOVQ runtime·faketime(SB), AX
 	CMPQ AX, $0
 	JEQ realtime
@@ -268,7 +268,7 @@ realtime:
 	RET
 
 TEXT syscall·now(SB),NOSPLIT,$0
-	JMP time·now(SB)
+	JMP runtime·walltime(SB)
 
 TEXT runtime·nacl_clock_gettime(SB),NOSPLIT,$0
 	MOVL arg1+0(FP), DI

src/runtime/sys_nacl_arm.s

@@ -196,7 +196,7 @@ TEXT runtime·mmap(SB),NOSPLIT,$8
 	MOVW	R0, ret+24(FP)
 	RET
 
-TEXT time·now(SB),NOSPLIT,$16
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	MOVW	$0, R0 // real time clock
 	MOVW	$4(R13), R1
 	NACL_SYSCALL(SYS_clock_gettime)
@@ -209,7 +209,7 @@ TEXT time·now(SB),NOSPLIT,$16
 	RET
 
 TEXT syscall·now(SB),NOSPLIT,$0
-	B time·now(SB)
+	B runtime·walltime(SB)
 
 TEXT runtime·nacl_clock_gettime(SB),NOSPLIT,$0
 	MOVW	arg1+0(FP), R0

src/runtime/sys_netbsd_386.s

@@ -134,8 +134,8 @@ TEXT runtime·setitimer(SB),NOSPLIT,$-4
 	INT	$0x80
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	LEAL	12(SP), BX
 	MOVL	$0, 4(SP)		// arg 1 - clock_id
 	MOVL	BX, 8(SP)		// arg 2 - tp

src/runtime/sys_netbsd_amd64.s

@@ -169,8 +169,8 @@ TEXT runtime·setitimer(SB),NOSPLIT,$-8
 	SYSCALL
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVQ	$0, DI			// arg 1 - clock_id
 	LEAQ	8(SP), SI		// arg 2 - tp
 	MOVL	$427, AX		// sys_clock_gettime

src/runtime/sys_netbsd_arm.s

@@ -137,8 +137,8 @@ TEXT runtime·setitimer(SB),NOSPLIT,$-4
 	SWI $0xa001a9	// sys_setitimer
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVW $0, R0	// CLOCK_REALTIME
 	MOVW $8(R13), R1
 	SWI $0xa001ab	// clock_gettime

src/runtime/sys_openbsd_386.s

@@ -140,8 +140,8 @@ TEXT runtime·setitimer(SB),NOSPLIT,$-4
 	INT	$0x80
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	LEAL	12(SP), BX
 	MOVL	$0, 4(SP)		// arg 1 - clock_id
 	MOVL	BX, 8(SP)		// arg 2 - tp

src/runtime/sys_openbsd_amd64.s

@@ -180,8 +180,8 @@ TEXT runtime·setitimer(SB),NOSPLIT,$-8
 	SYSCALL
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVQ	$0, DI			// arg 1 - clock_id
 	LEAQ	8(SP), SI		// arg 2 - tp
 	MOVL	$87, AX			// sys_clock_gettime

src/runtime/sys_openbsd_arm.s

@@ -150,8 +150,8 @@ TEXT runtime·setitimer(SB),NOSPLIT,$0
 	SWI	$0
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVW	CLOCK_REALTIME, R0	// arg 1 - clock_id
 	MOVW	$8(R13), R1		// arg 2 - tp
 	MOVW	$87, R12		// sys_clock_gettime

src/runtime/sys_plan9_386.s

@@ -102,8 +102,8 @@ TEXT runtime·nsec(SB),NOSPLIT,$8
 	MOVL	$-1, ret_hi+8(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	CALL	runtime·nanotime(SB)
 	MOVL	0(SP), AX
 	MOVL	4(SP), DX

src/runtime/sys_plan9_amd64.s

@@ -92,8 +92,8 @@ TEXT runtime·nsec(SB),NOSPLIT,$0
 	MOVQ	AX, ret+8(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	CALL	runtime·nanotime(SB)
 	MOVQ	0(SP), AX
 

src/runtime/sys_plan9_arm.s

@@ -139,7 +139,7 @@ TEXT runtime·nsec(SB),NOSPLIT,$-4-12
 	RET
 
 // time.now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$12-12
+TEXT runtime·walltime(SB),NOSPLIT,$12-12
 	// use nsec system call to get current time in nanoseconds
 	MOVW	$sysnsec_lo-8(SP), R0	// destination addr
 	MOVW	R0,res-12(SP)

src/runtime/sys_solaris_amd64.s

@@ -354,8 +354,8 @@ TEXT runtime·osyield1(SB),NOSPLIT,$0
 	CALL	AX
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	CALL	runtime·nanotime(SB)
 	MOVQ	0(SP), AX
 

src/runtime/sys_windows_386.s

@@ -432,8 +432,8 @@ TEXT runtime·switchtothread(SB),NOSPLIT,$0
 	MOVL	BP, SP
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	CALL	runtime·unixnano(SB)
 	MOVL	0(SP), AX
 	MOVL	4(SP), DX

src/runtime/sys_windows_amd64.s

@@ -465,8 +465,8 @@ TEXT runtime·switchtothread(SB),NOSPLIT|NOFRAME,$0
 	MOVQ	32(SP), SP
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	CALL	runtime·unixnano(SB)
 	MOVQ	0(SP), AX
 

src/runtime/time.go

@@ -301,3 +301,11 @@ func net_runtimeNano() int64 {
 func time_runtimeNano() int64 {
 	return nanotime()
 }
+
+var startNano = nanotime()
+
+//go:linkname time_now time.now
+func time_now() (sec int64, nsec int32, mono uint64) {
+	sec, nsec = walltime()
+	return sec, nsec, uint64(nanotime() - startNano + 1)
+}

src/time/export_test.go

@@ -30,4 +30,6 @@ func ResetZoneinfoForTesting() {
 var (
 	ForceZipFileForTesting = forceZipFileForTesting
 	ParseTimeZone          = parseTimeZone
+	SetMono                = (*Time).setMono
+	GetMono                = (*Time).mono
 )

src/time/format.go

@@ -424,8 +424,41 @@ func formatNano(b []byte, nanosec uint, n int, trim bool) []byte {
 
 // String returns the time formatted using the format string
 //	"2006-01-02 15:04:05.999999999 -0700 MST"
+// 
+// If the time has a monotonic clock reading, the returned string
+// includes a final field "m±<value>", where value is the monotonic
+// clock reading formatted as a decimal number of seconds.
 func (t Time) String() string {
-	return t.Format("2006-01-02 15:04:05.999999999 -0700 MST")
+	s := t.Format("2006-01-02 15:04:05.999999999 -0700 MST")
+
+	// Format monotonic clock reading as m=±ddd.nnnnnnnnn.
+	if t.wall&hasMonotonic != 0 {
+		m2 := t.ext
+		m1, m2 := m2/1e9, m2%1e9
+		if m2 < 0 {
+			m2 += 1e9
+			m1--
+		}
+		sign := byte('+')
+		if m1 < 0 {
+			sign = '-'
+			m1 = -m1
+		}
+		m0, m1 := m1/1e9, m1%1e9
+		var buf []byte
+		buf = append(buf, " m="...)
+		buf = append(buf, sign)
+		wid := 0
+		if m0 != 0 {
+			buf = appendInt(buf, int(m0), 0)
+			wid = 9
+		}
+		buf = appendInt(buf, int(m1), wid)
+		buf = append(buf, '.')
+		buf = appendInt(buf, int(m2), 9)
+		s += string(buf)
+	}
+	return s
 }
 
 // Format returns a textual representation of the time value formatted
@@ -1022,11 +1055,11 @@ func parse(layout, value string, defaultLocation, local *Location) (Time, error)
 
 	if zoneOffset != -1 {
 		t := Date(year, Month(month), day, hour, min, sec, nsec, UTC)
-		t.sec -= int64(zoneOffset)
+		t.addSec(-int64(zoneOffset))
 
 		// Look for local zone with the given offset.
 		// If that zone was in effect at the given time, use it.
-		name, offset, _, _, _ := local.lookup(t.sec + internalToUnix)
+		name, offset, _, _, _ := local.lookup(t.unixSec())
 		if offset == zoneOffset && (zoneName == "" || name == zoneName) {
 			t.setLoc(local)
 			return t, nil
@@ -1041,9 +1074,9 @@ func parse(layout, value string, defaultLocation, local *Location) (Time, error)
 		t := Date(year, Month(month), day, hour, min, sec, nsec, UTC)
 		// Look for local zone with the given offset.
 		// If that zone was in effect at the given time, use it.
-		offset, _, ok := local.lookupName(zoneName, t.sec+internalToUnix)
+		offset, _, ok := local.lookupName(zoneName, t.unixSec())
 		if ok {
-			t.sec -= int64(offset)
+			t.addSec(-int64(offset))
 			t.setLoc(local)
 			return t, nil
 		}

src/time/mono_test.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package time_test
+
+import (
+	"regexp"
+	"testing"
+	. "time"
+)
+
+func TestHasMonotonicClock(t *testing.T) {
+	yes := func(expr string, tt Time) {
+		if GetMono(&tt) == 0 {
+			t.Errorf("%s: missing monotonic clock reading", expr)
+		}
+	}
+	no := func(expr string, tt Time) {
+		if GetMono(&tt) != 0 {
+			t.Errorf("%s: unexpected monotonic clock reading", expr)
+		}
+	}
+
+	yes("<-After(1)", <-After(1))
+	ticker := NewTicker(1)
+	yes("<-Tick(1)", <-ticker.C)
+	ticker.Stop()
+	no("Date(2009, 11, 23, 0, 0, 0, 0, UTC)", Date(2009, 11, 23, 0, 0, 0, 0, UTC))
+	tp, _ := Parse(UnixDate, "Sat Mar  7 11:06:39 PST 2015")
+	no(`Parse(UnixDate, "Sat Mar  7 11:06:39 PST 2015")`, tp)
+	no("Unix(1486057371, 0)", Unix(1486057371, 0))
+
+	yes("Now()", Now())
+
+	tu := Unix(1486057371, 0)
+	tm := tu
+	SetMono(&tm, 123456)
+	no("tu", tu)
+	yes("tm", tm)
+
+	no("tu.Add(1)", tu.Add(1))
+	no("tu.In(UTC)", tu.In(UTC))
+	no("tu.AddDate(1, 1, 1)", tu.AddDate(1, 1, 1))
+	no("tu.AddDate(0, 0, 0)", tu.AddDate(0, 0, 0))
+	no("tu.Local()", tu.Local())
+	no("tu.UTC()", tu.UTC())
+	no("tu.Round(2)", tu.Round(2))
+	no("tu.Truncate(2)", tu.Truncate(2))
+
+	yes("tm.Add(1)", tm.Add(1))
+	no("tm.AddDate(1, 1, 1)", tm.AddDate(1, 1, 1))
+	no("tm.AddDate(0, 0, 0)", tm.AddDate(0, 0, 0))
+	yes("tm.In(UTC)", tm.In(UTC))
+	yes("tm.Local()", tm.Local())
+	yes("tm.UTC()", tm.UTC())
+	yes("tm.Round(2)", tm.Round(2))
+	yes("tm.Truncate(2)", tm.Truncate(2))
+}
+
+func TestMonotonicAdd(t *testing.T) {
+	tm := Unix(1486057371, 123456)
+	SetMono(&tm, 123456789012345)
+
+	t2 := tm.Add(1e8)
+	if t2.Nanosecond() != 100123456 {
+		t.Errorf("t2.Nanosecond() = %d, want 100123456", t2.Nanosecond())
+	}
+	if GetMono(&t2) != 123456889012345 {
+		t.Errorf("t2.mono = %d, want 123456889012345", GetMono(&t2))
+	}
+
+	t3 := tm.Add(-9e18) // wall now out of range
+	if t3.Nanosecond() != 123456 {
+		t.Errorf("t3.Nanosecond() = %d, want 123456", t3.Nanosecond())
+	}
+	if GetMono(&t3) != 0 {
+		t.Errorf("t3.mono = %d, want 0 (wall time out of range for monotonic reading)", GetMono(&t3))
+	}
+
+	t4 := tm.Add(+9e18) // wall now out of range
+	if t4.Nanosecond() != 123456 {
+		t.Errorf("t4.Nanosecond() = %d, want 123456", t4.Nanosecond())
+	}
+	if GetMono(&t4) != 0 {
+		t.Errorf("t4.mono = %d, want 0 (wall time out of range for monotonic reading)", GetMono(&t4))
+	}
+
+	tn := Now()
+	tn1 := tn.Add(1 * Hour)
+	Sleep(100 * Millisecond)
+	d := Until(tn1)
+	if d < 59*Minute {
+		t.Errorf("Until(Now().Add(1*Hour)) = %v, wanted at least 59m", d)
+	}
+	now := Now()
+	if now.After(tn1) {
+		t.Errorf("Now().After(Now().Add(1*Hour)) = true, want false")
+	}
+	if !tn1.After(now) {
+		t.Errorf("Now().Add(1*Hour).After(now) = false, want true")
+	}
+	if tn1.Before(now) {
+		t.Errorf("Now().Add(1*Hour).Before(Now()) = true, want false")
+	}
+	if !now.Before(tn1) {
+		t.Errorf("Now().Before(Now().Add(1*Hour)) = false, want true")
+	}
+}
+
+func TestMonotonicSub(t *testing.T) {
+	t1 := Unix(1483228799, 995e6)
+	SetMono(&t1, 123456789012345)
+
+	t2 := Unix(1483228799, 5e6)
+	SetMono(&t2, 123456789012345+10e6)
+
+	t3 := Unix(1483228799, 995e6)
+	SetMono(&t3, 123456789012345+1e9)
+
+	t1w := t1.AddDate(0, 0, 0)
+	if GetMono(&t1w) != 0 {
+		t.Fatalf("AddDate didn't strip monotonic clock reading")
+	}
+	t2w := t2.AddDate(0, 0, 0)
+	if GetMono(&t2w) != 0 {
+		t.Fatalf("AddDate didn't strip monotonic clock reading")
+	}
+	t3w := t3.AddDate(0, 0, 0)
+	if GetMono(&t3w) != 0 {
+		t.Fatalf("AddDate didn't strip monotonic clock reading")
+	}
+
+	sub := func(txs, tys string, tx, txw, ty, tyw Time, d, dw Duration) {
+		check := func(expr string, d, want Duration) {
+			if d != want {
+				t.Errorf("%s = %v, want %v", expr, d, want)
+			}
+		}
+		check(txs+".Sub("+tys+")", tx.Sub(ty), d)
+		check(txs+"w.Sub("+tys+")", txw.Sub(ty), dw)
+		check(txs+".Sub("+tys+"w)", tx.Sub(tyw), dw)
+		check(txs+"w.Sub("+tys+"w)", txw.Sub(tyw), dw)
+	}
+	sub("t1", "t1", t1, t1w, t1, t1w, 0, 0)
+	sub("t1", "t2", t1, t1w, t2, t2w, -10*Millisecond, 990*Millisecond)
+	sub("t1", "t3", t1, t1w, t3, t3w, -1000*Millisecond, 0)
+
+	sub("t2", "t1", t2, t2w, t1, t1w, 10*Millisecond, -990*Millisecond)
+	sub("t2", "t2", t2, t2w, t2, t2w, 0, 0)
+	sub("t2", "t3", t2, t2w, t3, t3w, -990*Millisecond, -990*Millisecond)
+
+	sub("t3", "t1", t3, t3w, t1, t1w, 1000*Millisecond, 0)
+	sub("t3", "t2", t3, t3w, t2, t2w, 990*Millisecond, 990*Millisecond)
+	sub("t3", "t3", t3, t3w, t3, t3w, 0, 0)
+
+	cmp := func(txs, tys string, tx, txw, ty, tyw Time, c, cw int) {
+		check := func(expr string, b, want bool) {
+			if b != want {
+				t.Errorf("%s = %v, want %v", expr, b, want)
+			}
+		}
+		check(txs+".After("+tys+")", tx.After(ty), c > 0)
+		check(txs+"w.After("+tys+")", txw.After(ty), cw > 0)
+		check(txs+".After("+tys+"w)", tx.After(tyw), cw > 0)
+		check(txs+"w.After("+tys+"w)", txw.After(tyw), cw > 0)
+
+		check(txs+".Before("+tys+")", tx.Before(ty), c < 0)
+		check(txs+"w.Before("+tys+")", txw.Before(ty), cw < 0)
+		check(txs+".Before("+tys+"w)", tx.Before(tyw), cw < 0)
+		check(txs+"w.Before("+tys+"w)", txw.Before(tyw), cw < 0)
+
+		check(txs+".Equal("+tys+")", tx.Equal(ty), c == 0)
+		check(txs+"w.Equal("+tys+")", txw.Equal(ty), cw == 0)
+		check(txs+".Equal("+tys+"w)", tx.Equal(tyw), cw == 0)
+		check(txs+"w.Equal("+tys+"w)", txw.Equal(tyw), cw == 0)
+	}
+
+	cmp("t1", "t1", t1, t1w, t1, t1w, 0, 0)
+	cmp("t1", "t2", t1, t1w, t2, t2w, -1, +1)
+	cmp("t1", "t3", t1, t1w, t3, t3w, -1, 0)
+
+	cmp("t2", "t1", t2, t2w, t1, t1w, +1, -1)
+	cmp("t2", "t2", t2, t2w, t2, t2w, 0, 0)
+	cmp("t2", "t3", t2, t2w, t3, t3w, -1, -1)
+
+	cmp("t3", "t1", t3, t3w, t1, t1w, +1, 0)
+	cmp("t3", "t2", t3, t3w, t2, t2w, +1, +1)
+	cmp("t3", "t3", t3, t3w, t3, t3w, 0, 0)
+}
+
+func TestMonotonicOverflow(t *testing.T) {
+	t1 := Now().Add(-30 * Second)
+	d := Until(t1)
+	if d < -35*Second || -30*Second < d {
+		t.Errorf("Until(Now().Add(-30s)) = %v, want roughly -30s (-35s to -30s)", d)
+	}
+
+	t1 = Now().Add(30 * Second)
+	d = Until(t1)
+	if d < 25*Second || 30*Second < d {
+		t.Errorf("Until(Now().Add(-30s)) = %v, want roughly 30s (25s to 30s)", d)
+	}
+
+	t0 := Now()
+	t1 = t0.Add(Duration(1<<63 - 1))
+	if GetMono(&t1) != 0 {
+		t.Errorf("Now().Add(maxDuration) has monotonic clock reading (%v => %v %d %d)", t0.String(), t1.String(), t0.Unix(), t1.Unix())
+	}
+	t2 := t1.Add(-Duration(1<<63 - 1))
+	d = Since(t2)
+	if d < -10*Second || 10*Second < d {
+		t.Errorf("Since(Now().Add(max).Add(-max)) = %v, want [-10s, 10s]", d)
+	}
+
+	t0 = Now()
+	t1 = t0.Add(1 * Hour)
+	Sleep(100 * Millisecond)
+	t2 = Now().Add(-5 * Second)
+	if !t1.After(t2) {
+		t.Errorf("Now().Add(1*Hour).After(Now().Add(-5*Second)) = false, want true\nt1=%v\nt2=%v", t1, t2)
+	}
+	if t2.After(t1) {
+		t.Errorf("Now().Add(-5*Second).After(Now().Add(1*Hour)) = true, want false\nt1=%v\nt2=%v", t1, t2)
+	}
+	if t1.Before(t2) {
+		t.Errorf("Now().Add(1*Hour).Before(Now().Add(-5*Second)) = true, want false\nt1=%v\nt2=%v", t1, t2)
+	}
+	if !t2.Before(t1) {
+		t.Errorf("Now().Add(-5*Second).Before(Now().Add(1*Hour)) = false, want true\nt1=%v\nt2=%v", t1, t2)
+	}
+}
+
+func TestMonotonicString(t *testing.T) {
+	t1 := Now()
+	re := regexp.MustCompile(` m=\+[0-9]+\.[0-9]{9}$`)
+	if !re.MatchString(t1.String()) {
+		t.Errorf("Now().String() = %q, want match for /%s/", t1.String(), re)
+	}
+
+	t2 := Now().Add(-5 * Hour)
+	re = regexp.MustCompile(` m=-[0-9]+\.[0-9]{9}$`)
+	if !re.MatchString(t2.String()) {
+		t.Errorf("Now().Add(-5*Hour).String() = %q, want match for /%s/", t2.String(), re)
+	}
+
+	t3 := Now().Add(1.2e18)
+	re = regexp.MustCompile(` m=\+120[0-9]{7}\.[0-9]{9}$`)
+	if !re.MatchString(t3.String()) {
+		t.Errorf("Now().Add(12e17).String() = %q, want match for /%s/", t3.String(), re)
+	}
+}

src/time/zoneinfo_plan9.go

@@ -95,7 +95,7 @@ func loadZoneDataPlan9(s string) (l *Location, err error) {
 
 	// Fill in the cache with information about right now,
 	// since that will be the most common lookup.
-	sec, _ := now()
+	sec, _, _ := now()
 	for i := range tx {
 		if tx[i].when <= sec && (i+1 == len(tx) || sec < tx[i+1].when) {
 			l.cacheStart = tx[i].when

src/time/zoneinfo_read.go

@@ -188,7 +188,7 @@ func loadZoneData(bytes []byte) (l *Location, err error) {
 
 	// Fill in the cache with information about right now,
 	// since that will be the most common lookup.
-	sec, _ := now()
+	sec, _, _ := now()
 	for i := range tx {
 		if tx[i].when <= sec && (i+1 == len(tx) || sec < tx[i+1].when) {
 			l.cacheStart = tx[i].when

src/time/zoneinfo_windows.go

@@ -132,7 +132,7 @@ func pseudoUnix(year int, d *syscall.Systemtime) int64 {
 			day -= 7
 		}
 	}
-	return t.sec + int64(day-1)*secondsPerDay + internalToUnix
+	return t.sec() + int64(day-1)*secondsPerDay + internalToUnix
 }
 
 func initLocalFromTZI(i *syscall.Timezoneinformation) {